1. Field of the Invention
The present invention relates to a system for providing a car navigation system with traffic information, and more particularly to a traffic information providing system suitable for providing traffic information on an area around a branch point such as an intersection or a junction on an expressway.
2. Related Background Art
For the purpose of reducing traffic congestion, there is known a service offered by the vehicle information and communication system (VICS) center to a vehicle driver to provide VICS information (traffic congestion or other traffic information) via a car navigation system. Moreover, there has been suggested a method of collecting traffic information using the VICS or the like (for example, refer to Japanese Laid-Open Patent Publication (Kokai) Nos. 2001-167385 and 9-318377).
By means of the VICS information obtained through the car navigation system, a vehicle driver can recognize, for example, that traffic congestion has occurred on a route that the vehicle is to pass through or can assess the time for passing through the route (the time required for traveling). Thereby, the vehicle driver can take measures to cope with the situation by changing the traveling route to his or her destination. Besides, the car navigation system can calculate a recommended route permitting the driver to avoid traffic congestion or the like on the basis of geographical information retained in the car navigation system and the VICS information given thereto and then to suggest it to the driver.
Incidentally, in the VICS information, a single piece of traffic information (for example, information such that the required time for traveling from a certain point A to a point B is a certain minutes) is assigned to a single route from the point A to the point B.
Meanwhile, if route “a” through the certain point A branches off in a plurality of routes “b” and “c” via a branch point X as shown in FIG. 2, the time required for traveling of the vehicle may depend upon whether the vehicle goes from the route a into the route b or into the route c. For example, in the shown example, the traffic lane of the route a toward the route b is in the traffic congestion, while the lane of the route a toward the route c is in a condition where vehicles are going smoothly. In this situation, the time required for the vehicle to go from the route a into the route b is longer than the time required for the vehicle to go into the route c.
The time required for traveling of the vehicle included in the traffic information on the route a provided by the VICS, however, is set to one value for the route a such as the time required for traveling on one of the lanes of the route a or an average of the time required for traveling on the lanes. In other words, a single piece of traffic information is assigned to the route a independently of which route is selected by the vehicle out of the routes a, b, and c.
Therefore, if the traffic congestion degree depends upon the route that the vehicle goes into from the route a as stated above, a large error often occurs in the time required for traveling determined from the VICS information. Moreover, if there are routes from the route a through the route b and from the route a through the route c to reach the final destination of the vehicle, the driver cannot determine which route is appropriate from the VICS information of the route a. As a result, occasionally the vehicle driver has selected an inappropriate route or the car navigation system has recommended an inappropriate route.